The present invention concerns improvements in the field of electronic musical instruments and, in particular, relates to an envelope modulator configured for imposing an envelope on a digitally generated musical waveform.
The tonal characteristics of the audible sounds produced by an electronic musical instrument are greatly enhanced by imposing a suitable envelope on the musical signal waveform produced by the tone generators of the instrument. In essence, a relatively constant level musical signal waveform is amplitude modulated in accordance with a selected envelope function in order to closely simulate the effects of an acoustical musical instrument. Most frequently, the modulation function includes a rising attack portion, which may be either linear or logarithmic, a constant level sustain portion and a falling decay portion which is usually logarithmic in nature and may have more than one decay rate.
In electronic musical instruments of the type having tone generation systems wherein a musical waveform is stored in a binary memory in sampled form and retrieved therefrom by a suitably arranged addressing system, the amplitude modulated waveform signal is normally derived by multiplying the stored waveform samples with a digitally stored representation of the desired envelope modulation. Conventional multiplication circuits used for this purpose require a number of clock periods corresponding to the bit size of the stored signals in which to complete a multiplication calculation. Since, in order to achieve adequate resolution, the bit size of the stored waveform and envelope samples is usually relatively large, e.g. on the order of 12 bits, it will be apparent that each multiplication calculation will require a relatively long period of time to complete. In view of the fact that signal processing in an electronic musical instrument must be carried out in real time, these long multiplication times are considered quite undesirable.
It is accordingly a basic object of the invention to provide an envelope generator for an electronic musical instrument adapted for imposing a selected envelope on a sequence of binary waveform samples, the envelope generator operating at a relatively fast speed without any degredation of accuracy in relation to slower prior art circuits.
It is a further object of the invention to provide such an envelope generator in which the step size of the output amplitude modulated binary signal is proportional to the signal level for maintaining the step noise nearly constant. Other objects of the invention include the provision of means allowing for adjustment of the initial gain or peak level of the envelope for controlling volume, adjustment of the attack and decay times of the envelope and adjustment of the character or shape of the envelope for creating different musical effects. Yet another object of the invention resides in the provision of an envelope generator wherein the attack and decay times are independent of the initial gain or peak level of the envelope modulation.